U.S. patent application number 12/084035 was filed with the patent office on 2009-06-18 for fabrication method of a mgb2 superconducting tape and wire.
Invention is credited to Hiroki Fujii, Hitoshi Kitaguchi, Hiroaki Kumakura, Takayuki Nakane.
Application Number | 20090156410 12/084035 |
Document ID | / |
Family ID | 37967733 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090156410 |
Kind Code |
A1 |
Nakane; Takayuki ; et
al. |
June 18, 2009 |
Fabrication Method of a MgB2 Superconducting Tape and Wire
Abstract
In a fabrication method of a MgB.sub.2 superconducting tape and
wire by filling a tube with a MgB.sub.2 superconducting powder and
forming it into a tape or wire, a fabrication method of a MgB.sub.2
superconducting tape (and wire) which is characterized by using a
MgB.sub.2 superconducting powder having a high critical current
density (J.sub.c) owing to its lowered crystallinity and having
potential for excellent grain connectivity as the MgB.sub.2
superconducting powder. Provided are a fabrication method of a
MgB.sub.2 superconducting tape and wire which can fabricate a
MgB.sub.2 superconducting tape and wire having a level of J.sub.c
sufficiently high for practical applications and homogeneous
quality throughout its length by an ex-situ process employing a
material of the composition suitable for its working environment as
the sheath material, and a MgB.sub.2 superconducting tape and wire
thereby fabricated.
Inventors: |
Nakane; Takayuki; (Ibaraki,
JP) ; Kitaguchi; Hitoshi; (Ibaraki, JP) ;
Fujii; Hiroki; (Ibaraki, JP) ; Kumakura; Hiroaki;
(Ibaraki, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
1030 15th Street, N.W.,, Suite 400 East
Washington
DC
20005-1503
US
|
Family ID: |
37967733 |
Appl. No.: |
12/084035 |
Filed: |
October 24, 2006 |
PCT Filed: |
October 24, 2006 |
PCT NO: |
PCT/JP2006/321174 |
371 Date: |
August 8, 2008 |
Current U.S.
Class: |
505/231 ;
174/125.1; 29/599; 505/230; 505/430 |
Current CPC
Class: |
Y10S 505/704 20130101;
C04B 2235/3826 20130101; H01L 39/2487 20130101; Y10T 29/49014
20150115; C04B 2235/421 20130101; C04B 2235/401 20130101; C01B
35/04 20130101; C04B 35/58057 20130101; C04B 2235/422 20130101 |
Class at
Publication: |
505/231 ;
505/430; 505/230; 29/599; 174/125.1 |
International
Class: |
H01B 12/10 20060101
H01B012/10; H01L 39/24 20060101 H01L039/24; H01B 12/02 20060101
H01B012/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 24, 2005 |
JP |
2005-309131 |
Claims
1. A fabrication method of a MgB.sub.2 superconducting tape and
wire by filling a tube with a MgB.sub.2 superconducting powder to
form it into a tape and wire, wherein a MgB.sub.2 superconducting
powder having its critical current density (J.sub.c) raised by its
lowered crystallinity and having high potential for excellent grain
connectivity is used as the MgB.sub.2 superconducting powder.
2 .A fabrication method of a MgB.sub.2 superconducting tape and
wire as set forth in claim 1, wherein a MgB.sub.2 superconducting
powder is obtained by mixing magnesium (Mg) or magnesium hydride
(MgH.sub.2) and boron (B) in a range of 0.9:2 to 1.1:2, filling a
tube with these powder mixture to form it into a tape and wire,
heat treating and making it into a powder form.
3. A fabrication method of a MgB.sub.2 superconducting tape and
wire as set forth in claim 2, wherein a powder of a ceramic
material containing carbon (C), a transition metal or an aromatic
organic compound is added to the powder mixture.
4. A fabrication method of a MgB.sub.2 superconducting tape and
wire as set forth in claim 3, wherein a powder of SiC or In
(indium) is added to the powder mixture.
5. A fabrication method of a MgB.sub.2 superconducting tape and
wire as set forth in claim 1, wherein the tube is a metal tube
containing one or more of the elements Fe, Cu, Al, Nb, Ti, Mg, Ag,
Au and Li.
6. A fabrication method of a MgB.sub.2 superconducting tape and
wire as set forth in claim 2, wherein the tube filled with the
powder mixture is an Fe tube.
7. A fabrication method of a MgB.sub.2 superconducting tape and
wire as set forth in claim 1, wherein a tube filled with the
MgB.sub.2 superconducting powder is an Al tube.
8. A fabrication method of a MgB.sub.2 superconducting tape and
wire as set forth in claim 1, wherein a multicore tape and wire is
formed by bundling a plurality of tubes filled with the MgB.sub.2
superconducting powder.
9. A fabrication method of a MgB.sub.2 superconducting tape and
wire as set forth in claim 1, wherein a tape and wire formed from a
tube filled with the MgB.sub.2 superconducting powder is heat
treated, if required.
10. A MgB.sub.2 superconducting tape and wire fabricated by a
method as set forth in claim 1.
11. A MgB.sub.2 superconducting tape and wire comprising an
elongated piece of MgB.sub.2 covered with a light-element metal as
the sheath material, and having a critical current density
(J.sub.c) of 900 A/cm.sup.2 or above at 10 T and 4.2 K.
12. A MgB.sub.2 superconducting tape and wire as set forth in claim
11, wherein the light-element metal is Al.
13. A MgB.sub.2 superconducting tape and wire as set forth in claim
11, wherein the MgB.sub.2 contains a powder of SiC or In
(indium).
14. A MgB.sub.2 superconducting tape and wire as set forth in claim
11, wherein the tape and wire is a multicore tape and wire made by
bundling a multiplicity of elongated pieces of MgB.sub.2 covered
with the sheath material.
15. A MgB.sub.2 superconducting tape and wire comprising a
multicore tape and wire formed by preparing a metal rod having a
plurality of bores and filling its bores with a raw material powder
like that used by a method as set forth in claim 1 to form a tape
and wire.
16. The articles made by employing the MgB.sub.2 superconducting
tape and wire as set forth in claim 10.
Description
TECHNICAL FIELD
[0001] The invention of the present application relates to a
fabrication method of a MgB.sub.2 superconducting tape and wire.
More particularly, it relates to a fabrication method for a
MgB.sub.2 superconducting tape and wire having a critical current
density (J.sub.c) sufficiently high for practical use by employing
an ex-situ powder in tube process which can impart homogeneous
quality throughout its length and employ a material with the
composition suitable for its working environment as the sheath
material, and a MgB.sub.2 superconducting tape and wire thereby
fabricated.
BACKGROUND ART
[0002] As magnesium diboride (MgB.sub.2), superconductor discovered
in Japan in 2001, has a superconducting critical temperature
(T.sub.c) of 39 K higher than that of any other metallic
superconductor and is relatively easy to form into a bulk material
or a tape (or wire), its physical properties and its formation into
a tape and wire are widely investigated throughout the world.
[0003] A powder-in-tube (PIT) method in which a metal tube (sheath
material) is filled with a raw material powder to form it into a
tape or wire is known as a principal method of forming a tape and
wire of a superconductor. The PIT method can be roughly classified
by a difference in raw material powder into an ex-situ process
utilizing a MgB.sub.2 superconducting powder itself, and an in-situ
process utilizing a powder mixture of e.g. a Mg powder and a B
powder and converting it into a superconductor by heat treatment
after forming a tape (or wire) shape. The ex-situ process has an
advantage over the in-situ process in fabricating a homogeneous
tape (and wire) and being suitable for the fabrication of a long
tape and wire. Moreover, while the in-situ process has no
alternative but to use as a sheath material e.g. iron or a nickel
alloy having no likelihood to react with the raw maternal powder at
the heat treatment, the ex-situ process, which makes it possible to
obtain superconducting performance without any heat treatment after
forming a tape (or wire), permits a broader range of free selection
for the sheath material and makes it possible to use some sheath
materials with the composition suitable for its working
environment, and its application is expected.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0004] However, the tape (and wire) fabricated by the ex-situ
process has never been of high critical current density (J.sub.c),
but a superconducting tape (and wire) obtained by employing a
MgB.sub.2 powder commonly used as a raw material has a very low
level of J.sub.c, which has been too low to be discussed for any
practical use. Accordingly, it is presently the case with the
ex-situ process that a large number of research attempts are under
way to achieve an improved J.sub.c. These attempts include a method
employing a sheath material of different composition and a method
giving heat treatment after forming a tape (or wire) which have
been found to be certainly effective for achieving an improving
J.sub.c, but these methods have been unable to make any effective
use of the inherent advantages of the ex-situ process featured by
the free selection of a sheath material and no necessity for heat
treatment. Neither these nor any other method has been able to
improve the J.sub.c of any MgB.sub.2 superconducting tape (or wire)
fabricated by the ex-situ process to a level equal to, or higher
than the J.sub.c of any tape (or wire) fabricated by the in-situ
process. Accordingly, in-situ process of PIT method is commonly
employed for the fabrication of a superconducting tape and
wire.
[0005] The invention of the present application has been made under
the circumstances stated above and is aimed at overcoming the
problems of the prior art and providing a fabrication method of a
MgB.sub.2 superconducting tape and wire which can fabricate a
MgB.sub.2 superconducting tape (and wire) having a level of J.sub.c
sufficiently high for practical uses and homogeneous quality
throughout its length by employing a material with suitable
composition for its working environment as the sheath material, and
fabricated MgB.sub.2 superconducting tape (ad wire) by this
method.
Means for Solving the Problems
[0006] In order to solve the problems stated above, the invention
of the present application firstly provides a fabrication method of
a MgB.sub.2 superconducting tape and wire which is characterized in
that a MgB.sub.2 superconducting powder having potential for
excellent grain connectivity and having high critical current
density (J.sub.c) owing to its lowered crystallinity is used as a
MgB.sub.2 superconducting powder for the ex-situ process in which a
tube is filled with a MgB.sub.2 superconducting powder to form it
into a tape or wire.
[0007] Secondly, it provides a fabrication method of a MgB.sub.2
superconducting tape and wire as set forth above, characterized in
that a MgB.sub.2 superconductor obtained by filling a tube with a
powder mixture of magnesium (Mg) or magnesium hydride (MgH.sub.2)
and boron (B) to form it into a tape (or wire), heating the tape
(or wire), and crushing the formed MgB.sub.2 superconductor into a
powder form. The ratio of Mg or MgH.sub.2 and B mixed to obtain the
MgB.sub.2 superconductor is in a Mg:B range of 0.9:2 to 1.1:2.
[0008] Thirdly, it provides a fabrication method of a MgB.sub.2
superconducting tape and wire as set forth above, characterized in
that a powder of a ceramic material including carbon (C), a
transition metal or an organic compound containing an aromatic
compound is added to the powder mixture.
[0009] Fourthly, it provides a fabrication method of a MgB.sub.2
superconducting tape and wire as set forth above, characterized in
that a powder of SiC or In (indium) is added to the powder
mixture.
[0010] Fifthly, it provides a fabrication method of a MgB.sub.2
superconducting tape and wire as set forth above, characterized in
that the tube is a metal tube containing one or more of the
elements Fe, Cu, Al, Nb, Ti, Mg, Ag, Au and Li.
[0011] Sixthly, it provides a fabrication method of a MgB.sub.2
superconducting tape and wire as set forth above, characterized in
that the tube filled with the powder mixture is an Fe tube.
[0012] Seventhly, it provides a fabrication method of a MgB.sub.2
superconducting tape and wire as set forth above, characterized in
that a tube filled with the MgB.sub.2 superconducting powder is an
Al tube.
[0013] Eighthly, it provides a fabrication method of a MgB.sub.2
superconducting tape and wire as set forth above, characterized in
that a multicore tape and wire are formed by e.g. bundling a
plurality of tubes filled with the MgB.sub.2 superconducting powder
or filling a metal rod having a plurality of bores with the raw
material powder.
[0014] Ninthly, it provides a fabrication method of a MgB.sub.2
superconducting tape and wire as set forth above, characterized in
that a tape or wire formed from a tube filled with the MgB.sub.2
superconducting powder is heated, if required.
[0015] According to a tenth aspect, it provides a MgB.sub.2
superconducting tape and wire characterized by being fabricated by
any of the methods as set forth above.
[0016] According to an eleventh aspect, it provides a MgB.sub.2
superconducting tape and wire characterized by comprising elongated
MgB.sub.2 covered with a light-element metal as the sheath
material, and having a critical current density (J.sub.c) of 900
A/cm.sub.2 or above at 10 T and 4.2 K.
[0017] According to a twelfth aspect, it provides a MgB.sub.2
superconducting tape and wire characterized in that the
light-element metal is Al.
[0018] According to a thirteenth aspect, it provides a MgB.sub.2
superconducting tape and wire characterized in that the MgB.sub.2
with SiC or In (indium) additives.
[0019] According to a fourteenth aspect, it provides a MgB.sub.2
superconducting tape and wire characterized in that the tape and
wire is a multicore tape and wire made by e.g. bundling a
multiplicity of elongated MgB.sub.2 wires covered with a sheath
material, or filling a metal rod having a plurality of bores with
the raw material powder to form a tape or wire.
[0020] According to a fifteenth aspect, it provides an article
characterized by employing any of the MgB.sub.2 superconducting
tape and wire as set forth above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 shows the full width of half maximum of the 110 peak
of X-ray diffraction patterns of a raw material powder and core of
Fe-sheathed MgB.sub.2 superconducting tape.
[0022] FIG. 2 is a schematic illustration of a fabrication process
for a MgB.sub.2 superconducting tape in Example 1.
[0023] FIG. 3 shows cross-sectional SEM images of Fe-sheathed
MgB.sub.2 superconducting tape made in Example 1.
[0024] FIG. 4 shows the J.sub.c-B characteristics of Fe-sheathed
MgB.sub.2 superconducting tapes made in Examples 1 and 2.
[0025] FIG. 5 shows the temperature dependence of the magnetic
irreversibility fields (B.sub.irr) of the Fe-sheathed MgB.sub.2
superconducting tapes made in Example 1.
[0026] FIG. 6 shows X-ray diffraction patterns showing the
crystallinity of MgB.sub.2 of Fe-sheathed MgB.sub.2 superconducting
tapes in Example 3.
[0027] FIG. 7 shows the AC magnetic field dependence of
superconducting transition curve of Fe-sheathed MgB.sub.2
superconducting tapes in Example 3.
[0028] FIG. 8 shows the magnetic field dependence of the normalized
pinning force of the MgB.sub.2 powder and Fe-sheathed MgB.sub.2
superconducting tapes in Example 3.
[0029] FIG. 9 shows the J.sub.c-B characteristics of MgB.sub.2
superconducting tapes in Example 4.
[0030] FIG. 10 shows comparison of J.sub.c-B characteristics
between Al-sheathed MgB.sub.2 superconducting tapes in Example 4
and that of the MgB.sub.2 tapes or wires known having highest
performance and fabricated by the ex-situ process.
[0031] FIG. 11 shows the AC magnetic field dependence of
superconducting transition of Al-sheathed MgB.sub.2 superconducting
tapes in Example 4.
[0032] FIG. 12 shows the magnetic field dependence of the
normalized pinning force of Al-sheathed and Fe-sheathed MgB.sub.2
superconducting tapes in Example 4.
[0033] FIG. 13 shows the J.sub.c-B characteristics in Example
5.
[0034] FIG. 14 shows the magnetic field dependence of resistive
voltage against applied current (160 mA) in Example 5.
[0035] FIG. 15 shows the J.sub.c-B characteristics in Example
6.
[0036] FIG. 16 shows the J.sub.c-B characteristics in Example
7.
BEST MODE FOR CARRYING OUT THE INVENTION
[0037] The invention of the present application has the features as
described above and the following is a description of modes of
carrying it out.
[0038] The fabrication method of a MgB.sub.2 superconducting tape
and wire which the invention of the present application provides is
a fabrication method of a MgB.sub.2 superconducting tape and wire
by the ex-situ process in which a tube is filled with a MgB.sub.2
superconducting powder to form a tape or wire, and characterized by
using a MgB.sub.2 superconducting powder having potential for
excellent grain connectivity and having high critical current
density (J.sub.c) owing to its low crystallinity.
[0039] A superconducting powder of high T.sub.c and excellent
crystallinity is generally considered preferable to use as a raw
material powder for fabricating a superconducting tape or wire of
high performance by the ex-situ process. However, the invention of
the present application employs as a raw material a powder of high
J.sub.c and high potential for excellent grain connectivity in
order to obtain a superconducting tape and wire of excellent
J.sub.c characteristics. The "high J.sub.c" and "high potential for
excellent grain connectivity" of the MgB.sub.2 superconducting
powder as the raw material can be understood as meaning that its
J.sub.c and grain connectivity are high as compared with an
ordinary MgB.sub.2 powder. When these characteristics are
evaluated, they can easily be compared by, for example, examining
the J.sub.c-B characteristics for the J.sub.c value and examining
the AC magnetic field dependence of superconducting transition
curve for the grain connectivity.
[0040] More specifically, the Mg powder used as the raw material by
the invention of the present application is characterized by its
relatively low crystallinity with, for example, a T.sub.c of about
36 K which is lower than the T.sub.c (about 39 K) of any known
ordinary MgB.sub.2 powder, as stated above. The low crystallinity
of the MB.sub.2 powder shortens its coherence length indicating the
spatial extension of the superconducting electron coupling, thereby
improving the critical value of the magnetic field which is
proportional to the reciprocal of the square of its coherence
length, while lowering its T.sub.c. As the improved critical value
of the magnetic field leads to an improvement of the maximum value
of the magnetic field allowing a superconductor to pass an applied
electric current, there apparently follows an improvement of
J.sub.c in a high magnetic field. According to the invention of the
present application, these characteristics are considered to
contribute greatly to improving the J.sub.c of the superconducting
tape and wire.
[0041] The MgB.sub.2 superconducting powder as the raw material can
be prepared by forming into a powder a MgB.sub.2 superconductor
obtained by filling a tube with a powder mixture of magnesium (Mg)
or magnesium hydride (MgH.sub.2) and boron (B) to form a tape and
heat treating it, as stated before. The MgB.sub.2 superconducting
powder can also be understood as a powder formed from the
superconducting core of a MgB.sub.2 tape or wire by the known
in-situ process.
[0042] As regards the powders of Mg or MgH.sub.2 and B, it is
possible to consider as a preferred example the use of, for
example, ones having a purity of 90% or above and an average
particle diameter of about 100 .mu.m or less so that the J.sub.c
and potential for excellent grain connectivity of the resulting raw
material powder may be excellent. It is considered that the higher
the purity, the better, while the smaller the particle diameter,
the more desirable. As a guide for the mixing ratio of Mg or
MgH.sub.2 and B, it is possible to try to obtain, for example, a
Mg:B ratio of, say, 0.9:2 to 1.1:2. The powder mixture of MgH.sub.2
and B is preferably stored in, for example, a vacuum or an inert
gas because of a low humidity and a low oxygen partial
pressure.
[0043] The various types tube for filling with the powder mixture
is possible to employ, if it is a tube which is neither broken in a
later tape (or wire)-forming process, nor reacts with the powder
mixture during heat treatment. The tube filled with the powder
mixture may, as a preferred example, be a metal tube containing one
or more of the elements Fe, Cu, Nb, Ti, Mg, Ag, Au and Li. In view
of the stress effect during the formation of a tape and wire, it is
more preferable to use e.g. an Fe or SUS tube.
[0044] While there is no particular limitation as to details of
technique for forming a tape or wire, it is preferable to employ a
technique which can give a satisfactorily high density to the
powder mixture in order to obtain a MgB.sub.2 superconducting raw
material powder with high J.sub.c and potential for excellent grain
connectivity. More specifically, it is, for example, preferable to
use a metal tube having as large a diameter as possible to increase
a cross-sectional reduction, as it is expected to achieve the
pulverization of particles, the preparation of a powder of high
density and a stress effect on MgB.sub.2. It is alternatively
possible to consider any other technique that is equally or even
more effective.
[0045] Heat treatment may be performed under conditions causing the
powder mixture to react and form MgB.sub.2. Referring to the
conditions for heat treatment, it is preferable to lower humidity
and oxygen partial pressure, and it will, for example, be possible
to realize an oxygen partial pressure of 10% or less. The lower the
oxygen partial pressure, the more desirable, and 1% or less is, for
example, preferable. Although there is no particular limitation as
to the temperature for heat treatment, it will, for example, be
possible to perform heating at a relatively low temperature not
exceeding 750.degree. C. There is no limitation as to time, but it
may be determined by taking the amount of the powder mixture, etc.
into account, and it will, for example, be suitable to perform heat
treatment at, say, 600-650.degree. C. for, say, an hour.
[0046] The MgB.sub.2 superconductor existing as a core is taken out
of the inside of the MgB.sub.2 superconducting tape (or wire)
thereby formed and is formed into a powder, whereby it is possible
to obtain a MgB.sub.2 superconducting powder as the raw material
for the invention of the present application. The MgB.sub.2
superconducting powder as the raw material may be of, say, 100
.mu.m or less, and the finer, the more desirable,
[0047] The fabrication method of a MgB.sub.2 superconducting tape
and wire according to the invention of the present application
forms a tape (or wire) by filling a tube with the MgB.sub.2
superconducting powder as described above and permits the use of
any of various kinds of tubes as the tube for filling with the
MgB.sub.2 superconducting powder if it is a tube of material which
is not broken at tape (or wire) forming process. It will, for
example, be possible to use a metal tube containing one or more of
the elements Fe, Cu, Al, Nb, Ti, Mg, Ag, Au and Li to make the tube
serve as a sheath material. In order to increase a cross-sectional
reduction and thereby obtain a MgB.sub.2 superconducting tape or
wire having still better characteristics, it is desirable to use as
a sheath material e.g. a high-strength material, such as stainless
steel of high hardness and rich ductility. The use of e.g. an Fe or
SUS tube facilitating a stress effect at the time of formation of a
tape or wire is shown as a more preferable example. On the other
hand, it is also possible to use a light materials, such as Al,
though it may be low in hardness and soft. The realization of high
characteristics by using Al as a sheath material for the MgB.sub.2
superconducting tape (and wire) has for the first time been made by
the invention of the present application.
[0048] Although there is no particular limitation as to details of
e.g. means or conditions for a tape (and wire) forming technique,
it is preferable to employ a technique giving a satisfactorily high
density to the raw material powder in order to obtain a MgB.sub.2
superconducting tape (or wire) having excellent J.sub.c
characteristics. More specifically, it will, for example, be
possible to perform a technique employing a metal tube having as
large a diameter as possible to realize an increased cross
sectional reduction, or employ a technique which is equally or
still more effective.
[0049] It is possible to form a multicore tape or wire by, for
example, bundling a plurality of tubes filled with the MgB.sub.2
superconducting powder after forming a wire in each tube and
inserting them into a tube. It is also possible to form a multicore
tape or wire by filling a metal tube having a plurality of bores
with the MgB.sub.2 superconducting powder and thereby forming a
tape or wire in each bore. There is, of course, no limitation as to
e.g. the material of the tubes in either case.
[0050] The fabrication method of a MgB.sub.2 superconducting tape
and wire according to the invention of the present application does
not necessarily require the heat treatment of the MgB.sub.2
superconducting powder as the raw material, but may include the
heat treatment of the MgB.sub.2 superconducting tape or wire as
formed in order to improve its desired characteristics, e.g. its
J.sub.c characteristics, to a still further extent. The conditions
for its heat treatment may, for example, include heating for, say,
0.01 to 100 hours at an oxygen partial pressure of 10% or less,
preferably 1% or less in a temperature range of, say,
600-900.degree. C. In this connection, it is necessary to choose
one not reacting with MgB.sub.2 as the tube to be filled with the
MgB.sub.2 superconducting powder in order to avoid the manufacture
of any tape and wire having poor characteristics.
[0051] The invention of the present application makes it possible
to add a powder of ceramics containing carbon, a transition metal
or an aromatic organic compound to the powder mixture when
preparing the MgB.sub.2 superconductor as the raw material powder.
Specific examples of the powders, which can be added, are a powder
of SiC and various kinds of transition metals including In, Sn, Sb,
Te, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ga, Ge, Sc, Ti, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, Pb, Bi, Pt, Hf, Ta and Hg.
[0052] It is also possible to mention as typical examples powders
of aromatic compounds, such as benzene, toluene, xylene,
naphthalene, perylene and anthracene. When a powder of SiC or In
is, for example, mixed, it is possible to add, say, 2.5 to 35 mol %
and preferably 10 to 25 mol % of a powder having an average
particle diameter of, say, 100 .mu.m or less and preferably, say,
10 nm to 1 .mu.m. As to the timing for the addition of the powder,
it does not matter whether it may be mixed with the starting
materials before MgB.sub.2 sintering, or it may be mixed with
MgB.sub.2 as prepared, if it precedes the formation of a tape and
wire by the ex-situ process, but in order to obtain still better
results, a compound containing carbon is preferably mixed with the
starting materials before MgB.sub.2 sintering, while a transition
metal, such as In, is preferably mixed with the MgB.sub.2 powder as
prepared. The addition of e.g. a SiC powder makes it possible to
improve the J.sub.c characteristics of the MgB.sub.2
superconducting tape and wire to a further extent even without the
heat treatment of the MgB.sub.2 superconducting tape and wire. It
follows that when a material reacting with MgB.sub.2 is used for
the tube, the use of the MgB.sub.2 superconducting powder mixed
with, for example, a SiC or In powder makes it possible to obtain a
tape or wire of high performance without any additional heat
treatment.
[0053] The fabrication method of a MgB.sub.2 superconducting tape
and wire as described above provides a superconducting tape and
wire having satisfactorily high J.sub.c characteristics even by the
ex-situ process. As it employs the ex-situ process, it is of great
significance from an industrial aspect, since it can fabricate a
tape and wire of homogeneous quality easily as compared with the
in-situ process and can fabricate an elongated tape and wire having
a length of 100 m or more, or even 1 km or more.
[0054] The MgB.sub.2 superconducting tape (and wire) provided by
the invention of the present application is fabricated by the
ex-situ process and realizes the tape (and wire) with excellent
characteristics in a magnetic field of high strength. For example,
an Fe-sheathed material ensures very excellent J.sub.c
characteristics including 1,000 A/cm.sup.2 or more at 4.2K and 12 T
and 3,000 A/cm.sup.2 or more at 4.2K and 10T. An Al-sheathed
material ensures excellent J.sub.c characteristics including 200
A/cm.sup.2 or more at 4.2K and 12 T and 900 A/cm.sup.2 or more at
4.2K and 10 T. The MgB.sub.2 superconducting tape and wire
according to the invention of the present application as described
is fabricated typically by the method of the invention as described
above.
[0055] Accordingly, the MgB.sub.2 superconductor in the tape or
wire is considered to have a coherence length shortened by its low
crystallinity and thereby an improved critical value of magnetic
field strength despite its lower T.sub.c. Its lower crystallinity
induces strain more likely to occur in the crystal during the
formation of a tape and wire by the ex-situ process, leading to a
shortened coherence length and thereby an improved critical value
of magnetic field strength. As its improved critical value of
magnetic field strength leads to an improvement in the maximum
value of magnetic field allowing the superconductor to pass an
applied electric current therethrough, it is considered that as a
result, its J.sub.c in high magnetic field is improved. The
MgB.sub.2 superconductor in the tape prepared by the invention of
the present application shows a full width of half maximum of
0.5.degree. or more at the 110 peak of its X-ray diffraction
pattern as shown in (b) of FIG. 1, and is characterized by showing
a definitely larger value than the value of, say, 0.4 of the
MgB.sub.2 superconductor in the tape and wire according to the
prior art. FIG. 1 shows the full width of half maximum at the 110
peak of (a) the raw material powder for the MgB.sub.2
superconducting tape prepared by the invention of the present
application, (b) heated core of MgB.sub.2 superconducting tape
according to the invention of the present application, and (c)
heated core of MgB.sub.2 superconducting tape prepared by the
ex-situ process of the prior art.
[0056] Moreover, the MgB.sub.2 superconducting tape and wire
according to the invention of the present application is an
elongated piece of MgB.sub.2 covered with e.g. Al as a sheath
material and is realized as one showing high superconducting
characteristics even in a magnetic field as high as 7 T or more,
for example, a fully practically acceptable level of, say,
1.0.times.10.sup.3 A/cm.sup.2 at 10 T and 4.2 K. Aluminum (Al) is a
material drawing attention as a sheath material for a
superconducting tape and wire owing to its good thermal
conductivity, low cost, high electrical conductivity and excellent
workability. However, Al and Mg easily react with each other and
melting point of Al is low, so that utilization of Al as the sheath
material is impossible for the in-situ process at the fabrication
of a MgB.sub.2 superconducting tape and wire. (Although an
experiment enabling it has recently been reported, it is still
considered as being practically impossible.) Although it is
possible to use Al as a sheath material in the ex-situ process, the
J.sub.c of the Al-sheathed MgB.sub.2 superconducting tape (and
wire) has been very low (80 A/cm.sup.2 at 4 T and 4.2 K). In view
of this, the MgB.sub.2 superconducting tape and wire according to
the invention of the present application, which employs Al as a
tube (sheath material) can be said to have realized an entirely
novel superconducting tape and wire.
[0057] Moreover, it is worthy of notice that Al, Mg and B are all
light elements. Al is one of few metals that are more suitable than
Fe, owing to its aspects such as lightweight, low induced
radioactivity, thermal conductivity, electrical conductivity,
magnetic property, workability and cost. MgB.sub.2 is the only
material that can be expected to form a tape or wire among the
superconductors consist of light elements, and the MgB.sub.2
superconducting tape and wire employing Al or any other
light-element metal as a sheath material can be expected to provide
a on-board magnet to be mounted on a rocket or Maglev train in view
of weight saving. The property of a light element with low induced
radioactivity can be expected to be application for a magnet of a
nuclear fusion reactor. A superconducting magnet formed from an Nb
compound is presently used as a magnet for plasma confinement for a
nuclear fusion reactor, but as Nb is a radioactive material, it is
imperative to store it for a long time in the order of several
hundred years before it can be disposed of after its use in e.g. a
nuclear fusion reactor. On the other hand, as the fabrication
technique for a MgB.sub.2 superconducting tape and wire according
to the invention of the present application can provide a tape and
wire for which a light-element metal with low induced radioactivity
is used as a sheath material, it can overcome this problem as a
magnet and is considered to be significant from an environmental
standpoint and from the standpoint of cost for e.g. storage.
[0058] The MgB.sub.2 superconducting tape and wire according to the
invention of the present application as described above may contain
a SiC powder added to MgB.sub.2, whereby its J.sub.c
characteristics are improved. It is not limited to a single-core
tape and wire, but can also be realized as e.g. a multicore tape
and wire having e.g. 10 to 1,000 or even 10,000 or more MgB.sub.2
superconducting cores.
[0059] Examples will now be shown to describe modes of carrying out
the invention of the present application in further detail. It is
needless to say that the present invention is not limited to the
following examples, but may include various modes varying from one
another in details.
EXAMPLES
Example 1
[0060] A MgB.sub.2 tape was fabricated in accordance with the step
chart shown in FIG. 2. A powder mixture obtained by mixing a
commercially available MgH.sub.2 powder (ABOGADO, 325 mesh, 96%
purity) and a commercially available powder of B (Sigma-Aldrich,
325 mesh, 99.99% purity) in a molar ratio of 1:2 was made to fill
an iron tube (sheath material) having an outside diameter of 6.02
mm and an inside diameter of 3.5 mm and formed into a tape having a
width of 5 mm and a thickness of 0.5 mm by groove-rolling and
flat-rolling forming. The tape was placed in an alumina boat,
covered with a titanium powder completely and heat treated at
600.degree. C. for an hour in an argon atmosphere by using a tube
furnace. In other words, the typical in-situ process of a MgB.sub.2
tape were performed. The MgB.sub.2 tape thereby obtained is called
tape (1).
[0061] Then, the sheath material was removed from the tapes (1) as
obtained above mechanically by using pincers or nippers, etc.,
whereby only the MgB.sub.2 formed inside was collected. The
collected MgB.sub.2 was crushed in a mortar to form a powder.
[0062] The powder as obtained was again filled in an iron tube
(sheath material) having an outside diameter of 6.02 mm and an
inside diameter of 3.5 mm and formed into a tape having a width of
5 mm and a thickness of 0.5 mm by groove-rolling and flat-rolling.
In other words, it can be considered that the typical ex-situ
process of a MgB.sub.2 tape were performed. The MgB.sub.2 tape
thereby obtained is called tape (2).
[0063] For comparison, a tape was made in the same way by employing
a MgB.sub.2 powder (Sigma-Aldrich, about 60 nm dia., 99.99% purity)
used typical ex-situ process and is tape (3).
[0064] Cross sectional images of Fe-sheathed MgB.sub.2
superconducting tape observed by a scanning electron microscope
(SEM) for (1), (2) and (3) are shown in (1), (2) and (3) of FIG. 3,
respectively.
[0065] The critical current density J.sub.c of each tapes (1), (2)
and (3) was measured in various magnetic fields at a liquid helium
temperature and the results are shown in FIG. 4. It was confirmed
that the tape (2) according to the invention of the present
application showed J.sub.c by far higher than that of the tape (3)
by the known ex-situ process. While the tape (3) by the known
ex-situ process did not show superconducting characteristics in a
high magnetic field over 7 T, the tape (2) according to the
invention of the present application showed excellent
characteristics as its J.sub.c value was comparable to that of the
tape (1) made by the in-situ process and even exceeded the J.sub.c
of tape (1) in a higher magnetic field.
[0066] The measurement for X-ray diffraction patterns for the
MgB.sub.2 cores of the tapes (1), (2) and (3) did not reveal any
impurity phase in any of them, but confirmed that the known tape
(3) was somewhat superior to the tape (2) according to the
invention of the present application in crystallinity. On the other
hand, the results for the measurement of the AC magnetic field
dependence of magnetization indicating the grain connectivity of
particles in the tapes shows that the tape (2) according to the
invention of the present application was by far superior to the
tape (3) in those characteristics.
[0067] It can be assumed from these results that the excellent
J.sub.c characteristics of the tape (2) according to the invention
of the present application are due to its magnetic and grain
connectivity improved by the use of a MgB.sub.2 powder of low
crystallinity and high J.sub.c as the raw material powder.
[0068] The tapes (1), (2) and (3) were also examined for the
temperature dependence of magnetic irreversibility field B.sub.irr
and the results are shown in FIG. 5. As the characteristics can be
considered superior toward the left top of the graph, the magnetic
irreversibility field characteristics of the tape (2) according to
the invention of the present application were superior to those of
the tapes (3) and (1) and showed that it was a material resisting a
lowering of J.sub.c irrespective of any change in temperature or
magnetic field.
Example 2
[0069] The tapes (2) and (3) of Example 1 were heat treated at
600.degree. C. to yield tapes (4) and (5), respectively. A tape (6)
was obtained by adding 7.5 mol % of a SiC nanopowder to the same
powder mixture of MgH.sub.2 and B powders as of Example 1 and
otherwise repeating the steps employed for the tape (4). The SiC
nanopowder was of Sigma-Aldrich, about 60 nm dia. and 99.99%
purity.
[0070] The J.sub.c-B characteristics of the tapes (4), (5) and (6)
at 4.2 K are shown in FIG. 4. The J.sub.c characteristics of the
tape (4) were by far better than those of the tape (2) and even
better than those of the tape (1) of Example 1 in a broad range of
magnetic fields higher than 8 T. On the other hand, the tape (5)
only showed substantially the same J.sub.c characteristics as the
tape (3). Although no additional heat treatment had been given to
the tape (6), it confirmed the possibility of obtaining excellent
J.sub.c-B characteristics comparable to those of the tape (4) which
had been heat-treated at 600.degree. C.
Example 3
[0071] The tapes (2) and (4) according to the invention of the
present application and the tape (1) by the known in-situ process
and the tapes (3) and (5) by the ex-situ process, which were the
same as those of Examples 1 and 2, were employed as samples and
their crystallinity was checked from X-ray diffraction patterns and
the results are shown in FIG. 6. The AC magnetic field dependence
of superconducting transition of each sample was evaluated by
measuring the temperature dependence of magnetization at different
AC magnetic fields and the results are shown in FIG. 7.
[0072] Although FIG. 6 does not show the presence of any impurity
in all MgB.sub.2 samples, the X-ray diffraction patterns of the
tapes (2) and (4) according to the invention of the present
application were found broad as a whole and the better
crystallinity of the known tapes (1), (3) and (5) was confirmed.
While a MgB.sub.2 superconducting tape is considered to have better
characteristics when grain aligned along the c-axis, the known
tapes (3) and (5) are apparently grain aligned along the c-axis,
since they showed a higher 002 peak than the tapes (2) and (4)
according to the invention of the present application.
[0073] FIG. 7 shows that the T.sub.c for all samples is lower than
the known T.sub.c (39 K) of MgB.sub.2. It, therefore, follows that
the MgB.sub.2 powder obtained from tape (1) and used as the
starting material for the invention of the present application does
not have any high T.sub.c. Such deterioration of T.sub.c is
considered to be due to the low sintering-level of the powder in
the case of the tape (1) and from the stress effect introduced at
the time of tape formation in the case of the tapes (2), (3), (4)
and (5). The T.sub.c of the tape (2) according to the invention of
the present application which had been made from the tape (1) was
found lower than that of the tape (3) formed from the commercially
available MgB.sub.2 powder.
[0074] On the other hand, it is generally considered that a sample
with superconducting transition curve broaden with a change in
applied magnetic field has poor gain connectivity. It was confirmed
that the MgB.sub.2 powder used as the starting material in the
invention of the present application had very excellent grain
connectivity, since it had the same physical properties as the tape
(1). The grain connectivity of the tape (2) according to the
invention of the present application which had been formed from
that MgB.sub.2 powder are considered as being superior to those of
the tape (3) formed from the commercially available MgB.sub.2
powder and further superiority of the tape (4) which had been given
additional heat treatment was confirmed.
[0075] FIG. 8 shows the magnetic field dependence of the normalized
pinning force of the tapes (1), (2) and (3). The magnetic field
dependence of the normalized pinning force indicates excellent
high-magnetic field characteristics when the peak of the curve is
located on the right side of the graph and it does not drop even in
a high magnetic field. FIG. 8 confirms that the tape (2) was
definitely superior to the tape (3) in pinning force
characteristics in a high magnetic field. This improvement in the
magnetic field dependence of the normalized pinning force was due
to an improved magnetic irreversibility field and the use of a
MgB.sub.2 powder of high J.sub.c as the raw material was found to
improve the magnetic irreversibility field of the tape over that of
any known tapes.
Example 4
[0076] An Al-sheathed MgB.sub.2 tape (7) according to the invention
of the present application and an Al-sheathed MgB.sub.2 tape (8) by
the known method were prepared by filling with the MgB.sub.2 powder
obtained from the tape (1) of Example 1 and with the MgB.sub.2
powder used in the known ex-situ process into the same shape of Al
tubes (sheath material) and by forming them into the tape,
respectively.
[0077] The J.sub.c-B characteristics of the tapes (7) and (8) are
shown in FIG. 9 with those of the Fe-sheathed tapes (1), (2) and
(3) of Example 1. It was confirmed that though the tapes (7) and
(8) having Al as the sheath material were both lower in J.sub.c-B
characteristics than the tapes (2) and (3) having Fe as the sheath
material, the Al-sheathed MgB.sub.2 tape (7) according to the
invention of the present application had by far higher
characteristics than the known Fe-sheathed MgB.sub.2 tape (3). It
was confirmed that the tape (7) showed a J.sub.c as high as
1.1.times.10.sup.4 A/cm.sup.2 at 4 T and 4.2 K and a practically
satisfactory level as high as 1.0.times.10.sup.3 A/cm.sup.2 (10 T
and 4.2 K) even in a high magnetic field over 7 T where the tape
(3) had not shown superconducting characteristics.
[0078] For reference, FIG. 10 compares the J.sub.c-B
characteristics of the tapes (7) and (8) with the characteristics
of the known highest quality tapes made by the ex-situ process.
When the Al-sheathed MgB.sub.2 tape (7) according to the invention
of the present application is compared with the ex-situ MgB.sub.2
tape having a sheath tube of other than Al, it is obvious that it
shows unparalleled characteristics in a high magnetic field for a
tape without heat treatment. In case of the comparison for
heat-treated tapes, it was comparable in characteristics in a high
magnetic field to those tapes of the highest quality.
[0079] FIG. 11 shows the AC magnetic field dependence of
superconducting transition of the tape (1) and the tapes (7) and
(8). FIG. 11 confirms that the tape (1) used as the starting
material for the tape (7) according to the invention of the present
application has very excellent grain connectivity, though it is not
an excellent material since its T.sub.c is high. It is obvious that
in the case of the Al-sheathed material, unlike the Fe-sheathed
material, the tape (7) is not particularly excellent, as it does
not substantially differ from the tape (8) in the grain
connectivity. This is apparently due to the fact that Al as a
sheath material is lower in strength than Fe, so that it cannot
density the superconducting core at the tape forming process.
[0080] FIG. 12 shows the results about the comparison for the
magnetic field dependence of the normalized pinning force between
the Al-sheathed tape (7), Fe-sheathed tapes (2) and (3) of Example
1. The magnetic field dependence of the normalized pinning force is
considered better when the peak of the curve is located on the
right side of the graph and does not drop even in a high magnetic
field, and it was confirmed that the tape (7) was definitely
superior in pinning force to the known Fe-sheathed tape, though it
was somewhat inferior in pinning force to the tape (2). This
improvement in the magnetic field dependence of the normalized
pinning force was due to an improved magnetic irreversibility field
and it indicates that to improve the magnetic irreversibility field
of the tape is improved even though the case employing Al as the
sheath material by utilizing the MgB.sub.2 powder with high J.sub.c
as the raw material.
Example 5
[0081] A tape with 10 mol % of SiC addition was fabricated by the
same way with the tape (6) of the invention of the present
application fabricated from the raw material powder with 7.5 mol %
of SiC powder addition in Example 2, and was heat treated at a
temperature of 700.degree. C. higher than the heat treatment
temperature employed for (6) to make a tape (9) according to the
invention of the present application.
[0082] The tape (9) was evaluated for J.sub.c-B characteristics.
The results are shown in FIG. 13 in comparison with those of the
tapes (2) and (4) according to the invention of the present
application, the ex-situ tape (5) by the known method (heated at
600.degree. C.) and the in-situ tape (1) which have been described
before.
[0083] As is obvious from FIG. 4, the J.sub.c of the tape (4) of
the invention of the present application obtained by heat treatment
at 600.degree. C. is higher than that of the tape (5) by the known
method and is higher than that of the in-situ tape (1) in a high
magnetic field over 8 T. It is obvious from FIG. 13 that the
J.sub.c of the tape (9) of the invention of the present application
obtained by heat treating the ex-situ tape formed from the
MgB.sub.2 powder prepared with SiC addition to the Mg and B
materials is still improved to a value of 9,000 A/cm.sup.2 or more
at 10 T.
[0084] FIG. 14 shows the results obtained by determining the
magnetic field dependence of resistive voltage (corresponding to
electric resistivity) against an applied electric current (160 mA)
for the tape (9), as well as the tapes (4), (5) and (1). When the
magnetic field is low, a superconducting state prevails and as
there is no electric resistance, there is no voltage. With an
increase in magnetic field, however, the superconducting state is
destroyed and there is a rise in voltage. The value of the magnetic
field at which the voltage rises from zero is the magnetic
irreversibility field, and the higher the magnetic irreversibility
field of a sample, the higher its J.sub.c is. According to FIG. 14,
the magnetic irreversibility of each tape is:
[0085] in-situ tape (1): 16.2 T
[0086] ex-situ tape (known method) (5): 13.8 T
[0087] ex-situ tape (new method) (4): 18.2 T
[0088] ex-situ tape (new method with SiC addition) (9); 20.2 T
and it is obvious that the tapes (4) and (9) according to the
invention of the present application are superior in magnetic
irreversibility field to the tape (5) by the known method and the
in-situ tape (1).
Example 6
[0089] A tape (10) according to the invention of the present
application was fabricated by adding In (10 mol %) to the MgB.sub.2
powder used as the raw material for the tape (2) according to the
invention of the present application and heating it at 600.degree.
C. as when the tape (4) had been made.
[0090] The tape (10) was evaluated for J.sub.c-B characteristics
and the results are shown in FIG. 15 in comparison with those of
the tapes (4), (5) and (1).
[0091] The tape (4) according to the invention of the present
application showed a J.sub.c higher than that of the in-situ tape
(1) in a high magnetic field over 8 T, but its less gradient in
J.sub.c-B characteristics made its J.sub.c lower than that of the
tape (1) in a lower magnetic field. However, it is obvious from
FIG. 15 that the ex-situ tape (10) made by In (10 mol %) addition
to the raw material powder showed a similar gradient in J.sub.c-B
characteristics to that of the in-situ tape (1) and had a high
J.sub.c even in a low magnetic field.
Example 7
[0092] The tapes according to the invention of the present
application were evaluated for the influence of different
conditions of heat treatment.
[0093] Tapes (11) and (12) according to the invention of the
present application were fabricated by the same way as the tape (4)
in Example 2, but by an hour of heat treatment at 600.degree. C.
and 10 hours of heat treatment at 300.degree. C., and the results
of their evaluation for J.sub.c-B characteristics are shown in FIG.
16 in comparison with those of the tape (2) without heat treatment
according to the invention of the present application.
[0094] It is obvious that the conditions of heat treatment are not
limited to a high temperature of 600.degree. C. or above as in the
case of any ordinary ex-situ tape, but that if the time for heat
treatment is prolonged, a temperature as low as 300.degree. C.
makes it possible to obtain the same J.sub.c-B characteristics as
those obtained by heating at 600.degree. C.
INDUSTRIAL APPLICABILITY
[0095] The fabrication method of a MgB.sub.2 superconducting tape
and wire according to the first invention of the present
application makes it possible to obtain even by the ex-situ process
a superconducting tape and wire having a satisfactorily high
J.sub.c characteristics, for example, a J.sub.c value improved by
one figure or more over the prior art. According to the second
invention, there is provided a method of preparing a MgB.sub.2
superconducting powder which is suitable for use in the ex-situ
process.
[0096] The fabrication method of a MgB.sub.2 superconducting tape
and wire according to the third and fourth inventions of the
present application makes it possible to improve various
characteristics of a MgB.sub.2 superconducting tape and wire even
without giving any heat treatment to the tape and wire.
[0097] Moreover, the fifth invention makes it possible to choose a
metal having the desired properties as the sheath material and the
sixth invention makes it possible to obtain a MgB.sub.2
superconducting powder of high quality which is low in reactivity
with a metal tube. The seventh invention makes it possible to
obtain a MgB.sub.2 superconducting tape and wire of high J.sub.c
characteristics having Al as the sheath material and explore a
drastically broad scope of use for a MgB.sub.2 superconducting tape
and wire.
[0098] The eighth invention makes it possible to fabricate a
MgB.sub.2 superconducting tape and wire in the form of any desired
multicore tape and wire, and the ninth invention makes it possible
to obtain a MgB.sub.2 superconducting tape and wire having further
improved characteristics by the heat treatment after tape (or wire)
forming process.
[0099] The tenth to fourteenth inventions realize the use of a
MgB.sub.2 superconducting tape and wire having excellent
characteristics as stated above and an Al-sheathed MgB.sub.2
superconducting tape and wire.
[0100] In addition, the fifteenth invention makes it possible to
realize any article that is useful in a variety of fields, for
example, in the form of a on-board magnet for a lightweight rocket,
or maglev train or a low-induced-radioactivity-magnet for the
plasma confinement of a nuclear fusion reactor.
* * * * *